Energy density is the number of calories (energy) in a given amount (volume) of food. By choosing foods that are low in calories, but high in volume, you can eat more and feel fuller on fewer calories. Fruits and vegetables are good choices because they tend to be low in energy density and high in volume.

So what about raisins? They're actually high in energy density — they pack a lot of calories into a small package. For example, 1/4 cup of raisins has about 100 calories. For about the same number of calories you could have 1 cup of grapes — and get more bite for your calorie buck.

Foods high in energy density include fatty foods, such as many fast foods, and foods high in sugar, such as sodas and candies. For example, a small order of fast-food fries has more than 200 calories.

For the same calories, you could have a heaping helping of fresh fruits and vegetables — such as this salad made with 10 cups of spinach, 1 1/2 cups of strawberries and a small apple. Plus with fresh fruits and vegetables, you get a plethora of valuable nutrients — not just empty calories. These foods also take longer to eat and are filling, which helps curb your hunger.

A single large glazed doughnut is generally around 300 calories. Will that keep you going all morning? For roughly the same number of calories, you could have a bowl of bran flakes with skim milk, blueberries and a slice of whole-wheat toast with peanut butter.

Tempted to grab a candy bar for lunch? For the same calories, you could choose a healthier and more filling option, such as pita stuffed with low-fat chicken salad.

Energy density is also important when you're snacking. If you're working on weight loss or maintenance, you want to keep your snacks at about 100 calories. One ounce of potato chips is about 150 calories. Or for only 100 calories, you could snack on 3 1/2 cups of air-popped popcorn.

To make this way of eating work for you, let your plate be your guide. Fill half of your plate with veggies, one-quarter with whole grains and the other quarter with a small serving of lean protein. This is a simple way to ensure you have filling, healthy meals.

Need a sweet ending? Try fresh fruit and yogurt as a tasty, low-calorie alternative to a slice of pie. The best part? This whole meal is less than 500 calories.